Nanofluids-based solar collectors as sustainable energy technology towards net-zero goal: Recent advances, environmental impact, challenges, and perspectives

Renewables are believed to be the future energy sources, given conventional energy sources' rapid depletion and environmental degradation. Among these, the ones used to harness solar energy have emerged as the most mature and promising. In these solar energy systems, nanofluids could be used as the conventional heat transfer fluid to overcome the low thermal conductivity of conventional fluids. This work gives an overview of the advancements made in the field of solar thermal collectors with a focus on nanofluids as heat transfer fluids for their performance enhancement. It also covers the description of the different solar collectors, including concentrated and non-concentrated solar collectors, and their potential applications in terms of space heating, hot water heating, space cooling, and industrial process heat. Environmental concerns about using nanofluids in these solar collectors are also considered. The impact of nanoparticles on friction factor, the outlet temperature of flowing fluid, optical properties, and pumping power is also briefly described. Different challenges such as production cost, volume and mass concentration factor, pressure drop, viscosity variation, and instability are investigated in detail. At the end of this study, future work is also proposed aiming to find suitable orientations for the next studies.